The Wnt pathway is an evolutionarily conserved growth pathway in multicellular organisms that regulates animal development and plays critical roles in human disease. Signaling through the Wnt pathway is regulated by secreted Wnt proteins, which act as morphogens to mediate 1) cell fate determination and differentiation required for establishing the body plan, neural patterning, and organogenesis, 2) cell motility and polarity, 3) cell proliferation and apoptosis, and 4) stem cell maintenance.
In Wnt signaling, the transcriptional coactivator, beta-catenin, is constitutively degraded in the absence of a Wnt signal thereby allowing a cell to maintain low cytoplasmic levels of beta-catenin and keeping the Wnt pathway in the “off” position. Degradation of beta-catenin requires its recruitment into a complex consisting primarily of Glycogen synthase kinase (Gsk3), Casein Kinase 1 (CK1), Protein phosphatase 2A (PP2A), Axin, and the tumor suppressor adenomatous polyposis coli (APC). Within this complex, beta-catenin is phosphorylated by CK1, which primes it for further phosphorylation by Gsk3. Phosphorylated beta-catenin is recognized by the SCF (Skip1, Cullen, F-box) ubiquitin ligase complex, of which the specificity F-box determinant is beta-TRCP, and targeted for polyubiquitination and subsequent degradation by the proteasome. The Wnt pathway is turned “on” upon binding of Wnt ligands to the Frizzled family of receptors and the coreceptor family members LDL receptor-related protein 5 or 6 (LRP5/6), which results in translocation of the beta-catenin destruction complex to the membrane through interaction of Axin with LRP5/6. The interaction between Axin and LRP5/6 is promoted by the phosphorylation of LRP5/6 by CK1 and Gsk3, and Axin-LRP5/6 interaction results in inhibition of beta-catenin phosphorylation and degradation. Because beta-catenin is continually synthesized in cells, its cytoplasmic concentration increases, and it enters the nucleus and forms a complex with the TCF/LEF1 family of transcriptional factors (as well as the nuclear proteins BCL9 and Pygopus) to regulate a Wnt-specific transcriptional program.
Our bodies are composed of numerous cell types specialized to perform specific functions. These specialized or differentiated cells are derived from a small group of stem and progenitor cells that have the capacity to divide asymmetrically, allowing them to regenerate themselves, and also giving rise to a daughter cell that can differentiate into cell types characteristic of various organs in our bodies. It is recognized that diseases like diabetes, Parkinson's disease, and heart disease are caused by death or dysfunction of differentiated cells in tissues where stem cells are limiting. These diseases may be caused by loss of stem cell activity and/or misregulation of critical signaling pathways in stem cells residing in tissues such as the pancreas, brain, and heart. The Wnt pathway is a key regulator of stem cell behavior and viability, and modulation of this pathway presents a method of treating diseases associated with dysfunctional stem cell activity. For example, activation of the Wnt pathway has been associated with heart failure, and inhibition of Wnt signaling has been shown to improve recovery after a heart attack in animal models. Thus, Wnt inhibitors could have broad applications in regenerative (stem cell) medicine for the treatment of major human diseases such as heart disease.
Cancer has been shown to be stem cell related disease, resulting from failure of cells to respond to normal cues to stop proliferating. Wnt signaling is also a critical pathway that drives the uncontrolled proliferation of many solid tumors in cancer stem cells (CSCs). Thus, therapies that down-regulate the activity of Wnt signaling, a fundamental pathway in CSCs, would be effective in the treatment of cancer. Such inhibitors would result in a long-term therapeutic benefit because the cells capable of repopulating the tumor would be killed. Most notably, there is clear evidence that colorectal cancer arises from mutations in the stem cell compartment, and it has been demonstrated that all major solid cancers in humans (e.g. melanoma, hepatocellular carcinoma, and breast cancer) have abnormal Wnt signaling. Thus, Wnt inhibitors may be useful in the treatment of most of the major solid cancers in humans.